1. Field of the Invention
This invention relates to an improved voltage comparator. More particularly, this invention relates to a voltage comparator having reduced settling time errors and thus reduced distortion.
2. Background of the Invention
There are many occasions throughout electronic engineering, and particularly in instrument manufacture, where it is necessary to compare a reference voltage to a signal voltage. For example, so-called "sampling comparators", as used in many oscilloscopes, measure the varying voltage of a signal to provide a suitable display thereof. Sampling voltage tracker circuits using an "equivalent-time" sampling approach are commonly used in such instruments. See for example U.S. Pat. No. 4,654,484 to Gyles, and "An 8-bit 200 MHz BiCMOS Comparator" Lim et al, IEEE J. of Solid-State Circuits, 25, 1, February 1990.
A block diagram of such a sampling comparator is shown in FIG. 1, and includes a time delay generator 10, a sampling comparator circuit 12, and a controller 14 with integral waveform display capability. The input signal to be displayed f(t) is supplied to the sampling comparator circuit 12 and to the time delay generator 10. Controller 14 determines a point on the input waveform f(t) to be measured. The sampling comparator circuit 12 generates a reference voltage and compares the reference voltage with the signal f(t) at the same point on each successive "copy" of the input signal. The comparison is made responsive to a strobe signal from the time delay generator 10. The sampling comparator circuit 12 thus provides an output signal to controller 14 responsive to the level of the waveform f(t) at the sampling time defined by the strobe signal. Controller 14 then instructs the time delay generator to alter the timing of the strobe signal, so that the sampling comparator similarly samples a subsequent point on the waveform. The process is repeated until the entire waveform has been sampled. At this point the complete series of values for the sampled points on the waveform are employed by controller 14 to reconstitute and display the waveform.
More specifically, the reference voltage is compared to the signal waveform repetitively at the same point in each cycle of the input waveform, when strobe pulses are received from a time delay generator 10. Over several cycles of operation, the reference voltage will closely approximate the input waveform f(t) at that particular point on the waveform. When successive comparison indicates that the reference voltage is substantially equal to the signal f(t), or after a fixed number of cycles of operation, controller 14 stores the reference voltage, and causes the time delay generator 10 to alter the timing of the strobe pulses so that the sampling takes place at a different point on the waveform. The process is then repeated, measuring a slightly different point on the waveform. When the entire waveform has been sampled at very short intervals, an accurate representation thereof can be generated and displayed.
FIGS. 2 and 3 illustrate two different embodiments of the sampling comparison circuit component 12 of the block diagram of FIG. 1. In FIG. 2, FIG. 2a shows a block diagram of the circuit, FIG. 2b shows the voltage of the signal f(t) being measured as a function of time, FIG. 2c shows the sequence of strobe pulses and FIG. 2d shows the reference signal, which converges to equal the value of the waveform f(t) at the strobe time. In the circuit of FIG. 2a, the signal to be measured is input to a comparator 20, together with a reference voltage provided by an integrator 22. The comparison of the signal and reference voltages occurs when a strobe signal pulse (FIG. 2c) is received. The successive outputs of the comparator 20 are summed in integrator 22, becoming the reference input. As indicated, a digital voltmeter 21 can be used to provide a digitized output signal, e.g. to controller 14 (FIG. 1).
As shown in FIG. 2b, the strobe pulses of FIG. 2c control the time at which the comparison between signal input and the reference input occurs. As shown in FIG. 2d the reference signal will typically oscillate slightly about the actual value of the signal f(t) at the strobe time.
FIG. 3 illustrates a further version of a sampling comparator circuit 12 which can be used in the instrument of FIG. 1. FIG. 3a is a block diagram of the circuit itself. FIG. 3b shows the signal voltage f(t) as a function of time. The signal f(t) is sampled at intervals defined by a strobe signal shown in FIG. 3c. FIG. 3d shows the reference signal. FIG. 3e shows the individual bits of a digital word representing the value of the reference voltage. One bit of this digital word is output upon each comparison.
Referring to FIG. 3a, comparator 20 compares the input signal f(t) to a reference signal upon receiving a strobe pulse (FIG. 3c) and outputs either a "one" or a "zero" bit responsive to the comparison. A successive approximation register (SAR) 23 receives the output bits, and provides a digital word as input to a digital-to-analog converter (DAC) 24. DAC 24 increments or decrements the reference voltage applied to the reference input of the comparator 20 responsive to the word provided by SAR 23 after the previous comparison. As shown in FIG. 3d, the change in the reference voltage responsive to each bit output by the successive approximation register has half the value of the change responsive to the preceding bit. Accordingly, the most significant bit of the word is output first, the second most significant bit is output next, and so on as indicated in FIG. 3e. Thus, at the conclusion of n sampling intervals, the digitized output provided by the successive approximation register 23 is an n-bit digital word directly representative of the value of the input signal at the strobe time.
Essentially the same comparator 20 is used in the circuits of both FIGS. 2 and 3. In the prior art, such comparators include two identical transistors, typically formed on the same substrate for uniformity, and connected differentially to a source of current. The reference voltage signal is applied to the base of a first reference transistor and the input signal to be measured is applied to the base of a second signal transistor. Accordingly, the amount of current conducted through the respective transistors can be measured and compared to determine whether the reference signal is greater than the sample signal or vice versa. Typically, the currents conducted through the two transistors are latched by a second pair of transistors responsive to the strobe pulse. See the Lim et al paper referred to above.
As discussed above, in the sampling circuits of FIGS. 2 and 3, the comparison of the reference voltage to the signal voltage takes place at a single instant during each period of the signal. Conventionally, however, the signal voltage and the reference voltage are provided to the bases of the signal and reference transistors of the comparator throughout the waveform, except at the strobe time, when the power is applied to the latching transistors. See Lim et al. Accordingly, the signal and reference transistors conduct varying amounts of current corresponding to the different levels of the input signal f(t) and the reference voltage. Therefore, differing quantities of heat are dissipated by the two transistors of the comparator. Accordingly, even though as noted both transistors are commonly formed on a single substrate, as they are heated differently their base-to-emitter voltage characteristics vary somewhat inconsistently,
Such differential heating effects are known to the art to cause a slight inaccuracy in the comparison and to distort the signal as finally represented by a series of words determined as above. For example, the "corners" of a square wave input signal f(t) tend to be rounded due to such differential heating effects. This distortion is referred to as a "thermal tail". U.S. Pat. No. 4,807,147 to Halbert et al, e.g., at Column 2, line 12, refers to such thermal tails. Halbert however does not provide any solution to thermal tails caused by the phenomenon just discussed. This distortion is particularly evident at input signal frequencies corresponding to the thermal time constants exhibited by the transistors of the comparator circuit.
The art suggests that such thermal tails may be compensated for in software; for example, controller 14 may be provided with software to correct the distortion induced by the thermal tails. However, such software compensation is complex and is only useful in a narrow range of frequencies.
The prior art also teaches preamplifying the input waveform f(t) and the reference signal using a second differential pair of transistors. Thermal tail phenomena are also understood to originate in such preamplifying transistors, again due to differential heating effects.